1. Field of the Invention
The present invention relates to an aberration amount detecting device, and a light receiving device. More particularly, the invention relates to a device for detecting light at positions before and after a focal point of condensed light.
2. Description of the Related Art
Nowadays, the number of types of optical recording media is increasing. The protecting layers having some different thicknesses are also provided. In reading out information from an optical disc, when a thickness of the protecting layer is deviated from a design value, or it is inclined to an optical axis of the object lens, a spherical aberration occurs in the wave front of an impinging light beam, and it hinders the image formation of a micro-spot.
Recently, some short-wave, high recording density optical heads are each designed to have a high numerical aperture of 0.85 or higher in order to reduce a diameter of a beam spot to read information. In this case, an aberration occurring in the optical system, in particular a spherical aberration caused by a thickness error of a protecting layer of the recoding medium greatly affects the image forming spot diameter. This aberration must be detected and corrected by some methods.
For this background, many aberration detecting methods have been proposed. JP-A-1998-214436 and JP-A-2000-57616 disclose the following technique. Light receiving elements are provided at a focal point. The light receiving elements are located at the inner and outer positions with respect to an optical axis of the beam. The light receiving areas of the inner and outer positions are each quartered by crossed division lines. By computing the output signals from those divided areas, a focal error signal and a spherical aberration signal are obtained. JP-A-2000-171346, JP-A-2002-39915, and JP-A-2002-55024 disclose another type of technique. Light receiving elements are provided at a point where light images. An incoming light beam is split into an inner light beam and an outer light beam. Those light beams are led to the light receiving elements, respectively. By computing the output signals from those divided areas, a focal error signal and a spherical aberration signal are obtained.
FIG. 1 is a diagram showing a light receiving device in use for the conventional aberration amount detecting mentioned above. A light source 103 generates laser light. Laser light emitted from the light source 103 travels through a predetermined path and reaches an optical disc (not shown). The lights reflected on the reflecting surface of the optical disc passes through a collimator lens 104. The reflected lights are incident on the areas of a hologram 101 and diffracted every reflected light, and image at predetermined positions on a light receiving element 102.
The reflected light that is incident on an inner peripheral area 101a of the hologram 101, images on a division line (not shown) provided on an inner peripheral light-amount detecting area 102a of the light receiving element 102. The reflected light incident on the outer peripheral area 101b of the hologram 101 images a division line (not shown) provided on an outer peripheral light-amount detecting area 102b of the light receiving element 102. The reflected lights that are incident on tracking error signal areas 101c and 101d image on tracking error signal detecting areas 102c and 102d, respectively. Paths of the reflected lights from the tracking error signal areas 101c and 101d to the tracking error signal detecting areas 102c and 102d, are not illustrated.
Already described, the inner and outer peripheral light-amount detecting area 102a and 102b are each divided into two sub areas by a division line. An amount of spherical aberration of the reflected light is computed according to the amounts of light from the two sub-areas and by using all the electric signals derived from the sub areas.
The aberration amount detecting device using the conventional light receiving device uses light in the vicinity of the image forming position to detect the aberration amount. For this reason, in a state that the aberration amount is almost zero, the reflected light must almost image at each light receiving area. The light receiving areas provided at least for detecting the aberration amount are each divided. It is necessary to position a microspot of which the diameter ranges from several μm to over ten μm on the division line at each light receiving area. Where each light receiving area is quartered by crossed division lines, it is necessary to position the center of the microspot at a nodal point of the crossed division lines.
Accordingly, it is difficult to set a positional relation between the hologram 101 and the light receiving areas of the light receiving element 102. In a case where the light receiving areas of the light receiving element 102 are integrally formed as shown, it is necessary to exactly set the orientations of the light receiving element as well as the position relation. This is very difficult to realize such by the manufacturing.
Since the spot diameter is extremely small, it is difficult to split the light containing the center of light from the peripheral light not containing the same, and apply those split lights to different light receiving areas. Even if the light is split into two different lights, and those split lights are successively received by the two light receiving areas, the number of divisions is at most two. Accordingly, many restrictions are imparted to the formulae for computing the aberration amount and the focus correction amount.
When device temperature varies, the frequency of the laser light slightly shifts from its correct value, a direction in which the laser light is diffracted by the hologram slightly changes, and hence the laser light lands at a position slightly different from the division line.
Also when the laser light used for the optical pickup used in the aberration amount detecting device is slightly shifted in the radial direction of the optical disc by the tracking servo, the laser light also lands at a position out of the division line.